Monroe Model K-121 Calculating Machine

This manual, full-keyboard non-printing modified stepped drum calculating machine has a metal frame painted black and a steel keyboard painted green. In the six columns of black and white color-coded keys, keys for odd digits are concave and those for even digits are relatively flat. At the bottom of each column is a red clearance key. Metal rods between the rows of keys serve as decimal markers.
Three key stems are in a column to the right of the number keys. One clears the entire keyboard. The other two are set to determine whether or not the keyboard clears after each calculation. The keys for these key stems are missing. A metal lever is to the right of the keyboard and a metal knob to the left. The operating crank on the right side rotates clockwise for addition and counterclockwise for subtraction.
The carriage in back of the keyboard has a row of 12 white numeral dials for recording results and a row of six numeral dials behind these in the revolution register. The revolution register has black digits for addition and red ones for subtraction. There is no carry in the revolution register. Two thin metal rods between the windows of these registers carry decimal markers. The carriage shift lever is at the front of the machine. To the right of the result register is a knob for lifting the carriage. A zeroing crank for dials on the carriage is on its right side. A bell rings when the result passes through zero (as in over-division). The machine has four rubber feet.
A mark on the front reads: MONROE. A mark on the back reads: MONROE (/) Calculating Machine Company (/) New York, U.S.A.. Stamped on the edge of the carriage on the back left is the serial number: 77811.
J. H. McCarthy, The Business Machines and Equipment Digest, 1928, pp. 9-29 to 9-33.
Currently not on view
Object Name
calculating machine
date made
Monroe Calculating Machine Company
Physical Description
plastic (overall material)
wood (overall material)
aluminum (overall material)
steel (overall material)
overall: 19.5 cm x 39 cm x 40 cm; 7 11/16 in x 15 11/32 in x 15 3/4 in
place made
United States: New Jersey, Orange
ID Number
catalog number
maker number
accession number
Science & Mathematics
Calculating Machines
See more items in
Medicine and Science: Mathematics
Calculating Machines
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Gift of Walton G. Banks, Inc.
Additional Media

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